<!DOCTYPE html>
<html lang="zh-cn">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <div id="app">
          <button @click="msg = '你坏'">修改msg</button>
          <button @click="msg = '你好坏'">修改msg第二次</button>
          <p>{{ msg }}</p>
    </div>
    <script src="./vue.js"></script>
    <script>
      new Vue({
        el: '#app',
        data () {
          return {
            msg: '你好'
          }
        },
        // 当msg变化了，就打印
        watch: {
            msg (newVal, oldVal) {
                console.log('msg变了', newVal, oldVal)
            }
        }
      })
    </script>
  </body>
</html>